Burnett, 36, has been on the disabled list since suffering a Grade 1 tear in his right calf last month. He threw a bullpen session to test the calf on Thursday. Burnett did not make any minor league rehab starts.
Burnett was 4-6 with a 3.12 ERA and 1.13 WHIP in 14 starts before the injury. His strikeout rate (9.96 K/9) is the fifth-highest in baseball.
Right-hander Jeanmar Gomez was scheduled to start Sunday and may go back to the bullpen. The Pirates have the best record in baseball and lead the St. Louis Cardinals by two games in the National League Central race entering this weekend's action.